In many clinical scenarios where access to hospital monitoring equipment is not available (including, e.g., home care, emergency care both in and out of hospitals, and screening/triage in remote locations) it is desirable to be able to check a subject's vital parameters. For example, when an unconscious person is found the presence or absence of circulation and ventilation should be assessed as soon as possible. It would also be desirable to quickly check vital parameters of subjects in hospital waiting rooms in order to identify which subjects are most in need of rapid treatment. At present there is limited technology available which can perform this function, and many existing technologies are expensive, difficult to use, and/or unreliable.
Prior art systems based on time-of-flight (TOF) measurements of reflected radio waves exist which can determine global body parameters (i.e. position and posture). However; these are typically unable to measure physical parameters inside of the body. For example, the WiZ system proposed by researchers at MIT is a multi-person centimeter-scale motion tracking system that pinpoints people's locations using only detected RF reflections from their bodies. WiZ can further track a person's breathing and heartbeat remotely, without requiring any body contact, by detecting the minute movements associated with breathing and heartbeat (e.g., the chest movements caused by the inhale-exhale process). It operates by transmitting a low-power wireless signal and monitoring its reflections. Distances to a reflecting object are measured using the TOF of the signal (i.e. the time it takes the signal to travel from the transmitter to a reflector and back). This system requires multiple transmit antennas and multiple receive antennas. Furthermore, because only a small portion of the 3D space into which signals are transmitted is modulated by the volume-of-interest, it is difficult to achieve good SNR and signal-to-crosstalk. The receiver must therefore be highly sensitive and have low phase noise. Because of these requirements, systems like WiZ are not suitable for use in most scenarios which occur outside the clinic where it is desired to measure vital parameters.
A quick and reliable means of checking the vital parameters of a subject would be a valuable tool to improve outcomes in such scenarios. There is therefore a need for an improved method and apparatus that can quickly provide a reliable estimate of one or more physiological characteristics of a subject.